A DFT Investigation of Alkyne Bromination Reactions Maxim V. Zabalov, Sergey S. Karlov,* Dmitri A. Lemenovskii, and Galina S. Zaitseva Chemistry Department, Moscow State University, B-234 Leninskie Gory, 119899 Moscow, Russia
[email protected] Received May 30, 2005
A DFT calculation study of the addition reaction between molecular bromine and the number of symmetrical or unsymmetrical substituted alkynes 1 (R-CtC-R′), where R ) R′ ) H (1a), Me (1b), t-Bu (1c), or Ph (1d), or R ) H and R′ ) Me (1e), t-Bu (1f), or Ph (1g), was performed. Two possible reactions were checked: (a) the reactions suitable for the gas-phase interactions, which start from a 1:1 Br2-alkyne π-complex and do not enter into a 2:1 Br2-alkyne π-complex; and (b) the processes passing through a 2:1 Br2-alkyne π-complex, which look more realistic for the reactions in solutions. The structures of the starting reactants and the final products as well as the possible stable intermediates have been optimized. The transition states of the predicted process have been found. Both trans- and cis-dibromoalkenes (2 and 3) may ensue without the formation of ionic intermediates from a π-complex of two bromine molecules with the alkyne (solution reactions). The geometry around the double bond forming in dibromoalkenes strongly depends on the nature of the substituents at the triple bond. The “cluster model” was also used for the prediction of solvent influence on the value of the activation barrier of the but-2-yne (1b) bromination reaction.
Introduction The addition reactions of halogens to compounds containing multiple bonds were the focus of researchers’ interest during the last century.1,2 The first reason for this interest was the possibility of using the products of these reactions as reactants for further organic syntheses. On the other hand, these processes were very attractive for use in theoretical studies as the model reactions for mechanistic investigations. It should be noted that among the halogenation processes, the reactions with bromine were studied the most often, probably because of the comparative ease of experimental work and the ability of the reactions to proceed under mild conditions. Special attention was devoted to the bromination of alkenes, whereas the reactions of allenes and alkynes are still not as well studied.3 According to the literature, different solvents have been used for alkyne bromination reactions, * Fax: +7 095/932 8846. (1) Ruasse, M.-F. Adv. Phys. Org. Chem. 1993, 28, 207-291. (2) Melloni, G.; Modena, G.; Tonellato, U. Acc. Chem. Res. 1981, 14, 227-233. (3) Lenoir, D.; Chiappe, C. Chem.sEur. J. 2003, 9, 1036-1044 and references therein.
such as acetic acid,4-7 alcohols,8 CHCl3,9 and ClCH2CH2Cl.10 The use of protic solvents leads to complicated mixtures of products, including ethers, esters, and ketones. Thus, the data obtained from the study of reactions in inert solvents such as haloalkanes are of great value for understanding the reaction pathways. The concept of bromination reactions with different organic alkynes can be considered complete because of the brilliant combined theoretical and experimental investigations of Bianchini and co-workers.10 In general, the reaction mechanism is similar to that for alkene bromination. The first step of the reaction is the formation of the Br2-alkyne π-complex, which transforms into (4) Pincock, J. A.; Yates, K. J. Am. Chem. Soc. 1968, 90, 5643-5644. (5) Pincock, J. A.; Yates, K. Can. J. Chem. 1970, 48, 3332-3337. (6) Schmid, G. H.; Modro, A.; Yates, K. J. Org. Chem. 1980, 45, 665667. (7) Yates, K.; Schmid, G. H.; Regulski, T. W.; Garratt, D. G.; Leung, H. W.; McDonald, R. J. Am. Chem. Soc. 1973, 95, 160-165. (8) Uemura, S.; Okazaki, H.; Okano, M.; Sawada, S.; Okada, A.; Kuwabara, K. Bull. Chem. Soc. Jpn. 1978, 51, 1911-1912. (9) Uemura, S.; Okazaki, H.; Okano, M. J. Chem. Soc., Perkin Trans. 1 1978, 1278-1282. (10) Bianchini, R.; Chiappe, C.; Lo Moro, G.; Lenoir, D.; Lemmen, P.; Goldberg, N. Chem.sEur. J. 1999, 5, 1570-1580.
10.1021/jo051085v CCC: $30.25 © 2005 American Chemical Society
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a 2:1 complex. The latter dissociates to a tribromide anion and one of two possible cationic intermediates, the cyclic bromirenium cation or the open bromovinyl cation. The attack of a [Br3] anion on cationic intermediates leads to the (E)-1,2-dibromoalkene or to the mixture of E and Z isomers. However, one question still remains undetermined: How do these cations form? In fact, according to data of the previous quantum chemical calculations,10,11 the processes of these cation formations are energetically unfavorable, because these reactions require the rupture of a Br-Br bond to form two ions from one neutral molecule (π-complex). The usual explanation for the formation of these cations is connected to the presence of a solvent in the reaction mixture, which may decrease the activation barrier because of some specific interactions.11 Although the latter suggestion is confirmed by the well-known fact that more polar solvents accelerate the reaction rate,12 no firm evidence based on calculations has been found to date. However, it should be noted that several works dealing with simulation of the solvent influence have been published (see below). The other problem still in question is which isomer, E or Z, will prevail in the concrete reaction. Very recently, we have studied the reactions of different elementsubstituted phenylacetylenes with bromine, where the element was silicon, germanium, or tin. The reactions were carried out in an aprotic solvent (CHCl3/CCl4), which led, in general, to the corresponding (Z)-1,2dibromoalkenes.13-15 This result cannot be explained on the basis of the generally accepted mechanism of alkyne bromination, in which appreciable amounts of E isomers should form. Thus, two open questions prompt us to find other possible pathways of the alkyne bromination. We have used the quantum chemical calculations for this purpose. According to our literature search, there is only one computational study16 of the bromination reaction of alkynes, which deals with calculations on the reaction products, transition structures, and intermediates of the bromination processes. Very recently, Herges et al. have published a study of the reaction of Br2 with C2H2 in which a nonionic mechanism was suggested (B3LYP/631G*, see below).16 Obviously, investigations of the mechanism of this process would be very important for understanding the bromination driving forces, including the reactions of alkynes with different structures. Modern quantum calculation methods, as well as the computer technique resources, enable us to examine the bromination reaction pathways in detail. It should be noted that besides the communication of Herges et al.16 several related works have been published (11) Chiappe, C.; De Rubertis, A.; Detert, H.; Lenoir, D.; Wannere, C. S.; Schleyer, P. von R. Chem.sEur. J. 2002, 8, 967-978. (12) Selina, A. A.; Karlov, S. S.; Zaitseva, G. S. Vestn. Mosk. Univ., Ser. 2: Khim. 2004, 45, 147-171. (13) Karlov, S. S.; Shutov, P. L.; Churakov, A. V.; Lorberth, J.; Zaitseva, G. S. J. Organomet. Chem. 2001, 627, 1-5. (14) Selina, A. A.; Karlov, S. S.; Harms, K.; Tyurin, D. A.; Oprunenko, Yu. F.; Lorberth, J.; Zaitseva, G. S. Z. Naturforsch. 2003, 58b, 613619. (15) Selina, A. A.; Karlov, S. S.; Gauchenova, E. V.; Churakov, A. V.; Kuz’mina, L. G.; Howard, J. A. K.; Lorberth, J.; Zaitseva, G. S. Heteroat. Chem. 2004, 15, 43-56. (16) Herges, R.; Papafilippopoulos, A.; Hess, K.; Chiappe, C.; Lenoir, D.; Detert, H. Angew. Chem. 2005, 117, 1437-1441; Angew. Chem., Int. Ed. 2005, 44, 1412-1416.
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to date. Yamabe et al.17 have studied the electrophilic addition reactions of the molecular halogens (F2, Cl2, and Br2) with ethene. They have found that the bromination of C2H4 proceeds through a zwitterionic three-centered transition state (cyclic bromonium ion plus Br-) and leads to the anti addition product, with an activation barrier of 61.4 kcal mol-1. Very recently, Kurosaki18,19 suggested a radical mechanism of gas-phase ethylene chlorination. He has studied two different pathways of the reaction, i.e., direct Cl2 addition to C2H4 and Cl abstraction from Cl2 by C2H4. It has been found that in the gas phase, the C2H4 + Cl2 f C2H4Cl + Cl (asymptote without undergoing a TS, 25.1 kcal mol-1 higher in energy than the reactants) f C2H4Cl2 pathway is energetically more feasible than the C2H4 + Cl2 f TS (36.3 kcal mol-1) f C2H4Cl2 pathway. De Almeida et al.20 have found the activation barrier of the analogous acetylene reaction leading to cis- and trans-C2H2Cl2 to be 22.6-41.8 kcal mol-1, depending on the calculation method and basis set used. The reaction pathway includes the initiation step C2H2 + Cl2 f C2H2Cl + Cl, the isomerization step transC2H2Cl T cis-C2H2Cl, three different propagation steps, and three different termination steps. The analysis of the kinetics data gives 97.3% trans product and 2.7% cis product at room temperature. However, a radical mechanism of the halogenation of alkynes, obviously possible in the gas phase at high temperature or with UV irradiation, seems to be unlikely in solution. With regard to the influence of a solvent, Rivail and co-workers21 employed a continuum model for analysis of the bromination reaction of ethylene. They found a considerable decrease in the activation barrier when carried out in the solvent (0 ) 78.4) in comparison with that in the gas phase. Similar results have been obtained by Cossi et al.22 Kurosaki23 has studied the chargeseparation process of the C2H4 + Cl2 reaction in water using a cluster model (four H2O molecules). A very low activation barrier of such separation has been found. However, the final cluster still contained Cl‚‚‚Cl contacts, and no IRC analysis leading to the chlorination products has been done. Additionally, the recent work of Coxon and Smith24 examined the possible transition states of cyclopropane bromination. This process does not require the heterolytic cleavage of the bromine molecule during the course of the reaction. Experimental results for acetylene (1a) bromination are ambiguous. Three different products were reported, but their structures were not determined. The authors only supposed the formation of cis- and trans-1,2-dibromoethylene as well as 1,1,2,2-tetrabromoethane at room temperature.25 It is also known that the preparation of (17) Yamabe, S.; Minato, T.; Inagaki, S. J. Chem. Soc., Chem. Commun. 1988, 532. (18) Kurosaki, Y. J. Mol. Struct. (THEOCHEM) 2000, 503, 231240. (19) Kurosaki, Y. J. Mol. Struct. (THEOCHEM) 2001, 545, 225232. (20) Resende, S. M.; Pliego, J. R., Jr.; De Almeida, W. B. J. Chem. Soc., Faraday Trans. 1998, 94, 2895-2900. (21) Assfeld, X.; Garapon, J.; Rinaldi, D.; Ruiz-Lo´pez, M. F.; Rivali, J. L. J. Mol. Struct. (THEOCHEM) 2001, 545, 225-116. (22) Cossi, M.; Persico, M.; Tomasi, J. J. Am. Chem. Soc. 1994, 116, 5373-5378. (23) Kurosaki, Y. J. Phys. Chem. A 2001, 105, 11080-11087. (24) Coxon, J. M.; Smith, W. B. J. Org. Chem. 2000, 65, 2192-2194. (25) Charlson, A. J.; Shapiro, J. S.; Ware, G. W.; Watton, E. C. J. Chem. Educ. 1978, 55, 338-339.
DFT Investigation of Alkyne Bromination Reactions SCHEME 1. Possible Products of Alkyne Bromination
for the prediction of solvent influence on the value of the activation barrier for the but-2-yne (1b) bromination reaction.
Methods of Calculations
pure trans-1,2-dibromoethylene from bromine addition to acetylene requires the presence of CuBr2 to prevent cis isomer formation.26 Uemura et al.9 have found the formation of both 2g (see Scheme 1 for definitions) and 3g (85/15 ratio) in the reaction of 1g in chloroform (the kinetic control conditions: 20 °C, 0.5 h). The treatment of hexyne-1 (analogue of 1e) and hexyne-3 (the analogue of 1b) in the same conditions led to mixtures of corresponding E and Z isomers with 96/4 and 100/0 ratios, respectively. Bianchini and co-workers10 have reported that bromination of 1d, 1g, and hexyne-2 (the closely related analogue of 1b) in 1,2-dichloroethane at room temperature gave mixtures of corresponding cis- and trans-dibromides with E/Z ratios ) 60/40, 57/43, and 100/ 0, respectively. Thus, the publication of Herges’ results16 motivates us to report the results of our DFT study of the addition reaction between molecular bromine and a number of symmetrical or unsymmetrical substituted alkynes 1 (RCtC-R′), where R ) R′ ) H (1a), Me (1b), t-Bu (1c), or Ph (1d), or R ) H and R′ ) Me (1e), t-Bu (1f), or Ph (1g). The substituents were chosen for their different steric and electronic properties, as well as the opportunity to compare our calculated results with experimental data. Besides the bromination reactions leading to 1,2-dibromoalkenes, we also analyzed other possible processes, the formation of 1,1-dibromoalkenes and the cleavage of 1,2and 1,1-dibromoalkenes. There are several previous theoretical works27-32 that dealt with the elimination reactions of dihaloalkenes (see below). We have studied two possible pathways: (a) reactions starting from a 1/1 Br2-alkyne π-complex and not forming a 2/1 Br2-alkyne π-complex and (b) processes passing through a 2/1 Br2-alkyne π-complex. The first reaction is suitable for the gas-phase interactions, whereas the second looks more realistic for the processes in solutions. The structures of the reactants and the products, as well as the possible stable intermediates, have been optimized. The transition states of the predicted process have been found. The cluster model was also used (26) Belohlaw, L. R.; McBee, E. Ind. Eng. Chem. 1961, 53, 10151024. (27) Resende, S. M.; De Almeida, W. B. Chem. Phys. 1998, 238, 1120. (28) Riehl, J.; Musaev, D. G.; Morokuma, K. Chem. Phys. 1994, 101, 5942-5956. (29) Martinez-Nuo`ez, E.; Va´zquez, S. J. Chem. Phys. 2004, 121, 5179-5182. (30) Gonza´lez-Va´zquez, J.; Ferna´ndez-Ramos, A.; Martinez-Nunez, E.; Va´zquez, S. A. J. Phys. Chem. A 2003, 107, 1389-1397. (31) Gonza´lez-Va´zquez, J.; Martinez-Nunez, E.; Ferna´ndez-Ramos, A.; Va´zquez, S. A. J. Phys. Chem. A 2003, 107, 1398-1404. (32) Martinez-Nunez, E.; Va´zquez, S. A.; Aoiz, F. J.; Banares, L.; Castillo, J. F. Chem. Phys. Lett. 2004, 386, 225-232.
Theoretical calculations were performed by the density functional theory (DFT) using the ab initio generalized gradient approximation and the PBE functional33,34 implemented in the program “Priroda”.35 The one-electron wave functions were expanded in the extended TZ2P atomic basis sets of the contracted Gaussians (5s1p):[3s1p] for H, (11s6p2d):[4s3p2d] for C, and (19s16p9d):[6s5p3d] for Br. Full geometry optimization was performed for all stable compounds, and a saddle point search was performed for transition states. The type of stationary points was confirmed by vibrational frequency calculations; saddle points were confirmed also by the intrinsic reaction coordinate (IRC) calculation. The present method has been used and has given very useful results, including the study of bromine-containing molecules.14,36-38
Results and Discussion Analysis of Final Product Structures. It is wellknown that bromination reactions of alkyne with reagent ratio 1/1 may lead to (E)- or (Z)-dibromoalkenes (Scheme 1). One can expect that several other products may also form in the course of the reaction because of the rearrangement or cleavage processes. We have calculated the sums of the total energies of the reactants (1 + Br2), and the differences between these values and the total energies of the optimized dibromoalkenes 2-4 or the cleavage products (5 + 6) and (7 + 8), which are summarized in Table 1 (see also Figure 1). Optimized structural parameters of 1-8 are available as Supporting Information. Their geometries are as expected, and are consistent with the available gas electron diffraction and microwave spectroscopy data.39-42 The most thermodynamically stable products for the bromination reaction of the alkynes 1d, 1f, and 1g are the corresponding cis-dibromides 3d, 3f, and 3g. However, trans-alkenes are the global minima for the bromination reactions of acetylenes (1a-c,e). It should be noted that the presence of one or two phenyl groups in the alkynes, as well as one bulky t-Bu group, stabilizes the cis geometry of dibromoadducts. However, the values of the total energy of corresponding cis and trans derivatives are similar (except 1c and 1f, in which the steric crowding of t-Bu groups strongly affects the total energy (33) Perdew, J. P.; Burke, K.; Ernzerhoff, M. Phys. Rev. Lett. 1996, 77, 3865-3868. (34) Ernzerhoff, M.; Scuseria, G. E. J. Chem. Phys. 1999, 110, 50295036. (35) Laikov, D. N. Chem. Phys. Lett. 1997, 281, 151-156. (36) Shutov, P. L.; Karlov, S. S.; Harms, K.; Tyurin, D. A.; Sundermeyer, J.; Lorberth, J.; Zaitseva, G. S. Eur. J. Inorg. Chem. 2004, 2498-2503. (37) Karlov, S. S.; Yakubova, E. Kh.; Gauchenova, E. V.; Selina, A. A.; Churakov, A. V.; Howard, J. A. K.; Tyurin, D. A.; Lorberth, J.; Zaitseva, G. S. Z. Naturforsch. 2003, 58b, 1165-1170. (38) Shutov, P. L.; Karlov, S. S.; Harms, K.; Tyurin, D. A.; Churakov, A. V.; Lorberth, J.; Zaitseva, G. S. Inorg. Chem. 2002, 41, 6147-6152. (39) Abramenkov, A. V.; Almenningen, A.; Cyvin, B. N.; Cyvin, S. J.; Jonvik, T.; Khaikin, L. S.; Roemming, C.; Vilkov, L. V. Acta Chem. Scand. A 1988, 42, 674-681. (40) Schultz, G.; Nagy, T.; Portalone, G.; Ramondo, F.; Hargittai, I.; Domenicano, A. Struct. Chem. 1993, 4, 183-190. (41) Berry, R. J.; Harmony, M. D. Struct. Chem. 1990, 1, 49-59. (42) Le Guennec, M.; Demaison, J.; Wlodarczak, G.; Marsden, C. J. J. Mol. Spectrosc. 1993, 160, 471-490.
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Zabalov et al. TABLE 1. Total Energies of the Reactants 1 + Br2 (hartree), and the Relative Energies (kcal mol-1) of the Possible Productsa a b c d e f g
R
R′
1 + Br2 (Etotal)
2 (∆E)
3 (∆E)
4 (∆E)
5 + 6 (∆E)
7 + 8 (∆E)
H Me t-Bu Ph H H H
H Me t-Bu Ph Me t-Bu Ph
-5224.89661 -5303.46460 -5539.10285 -5686.61052 -5264.18180 -5382.00087 -5455.75303
-44.06 -38.18 -16.65 -27.60 -41.31 -34.19 -37.01
-43.86 -35.60 -8.33 -28.13 -40.34 -38.28 -38.09
-41.85 -33.73 2.79 -24.00 -37.84 -33.83 -35.74
-2.13b 6.54b 1.07b 10.86b -1.87 -2.02 -2.30
-2.13b 6.54b 1.07b 10.86b 7.78 2.40 10.36
a The value of the relative energy is the corresponding difference: E b total(corresponding molecule) - Etotal(1 + Br2). Values for 5 + 6 and 7 + 8 are identical.
TABLE 2. Calculated Relative Energies (∆E, kcal mol-1) of Br2-Alkyne π-Complexes (π-C-I) R R′ ∆E
FIGURE 1. Calculated geometries of compounds 1-8 involved in the study (general view).
FIGURE 2. Calculated geometries of π-complexes π-C-I derived from symmetrical and unsymmetrical alkynes (general views).
of dibromoalkenes), and the formation of both isomers is thermodynamically expected. Thus, the kinetic factors probably determine the product distribution. The values of total energy of 1,1-dibromides 4 are, in general, slightly greater than those for both 2 and 3. However, both possible cleavage processes (5 + 6 and 7 + 8) are strongly unfavorable. Analysis of the Alkyne (1)-Br2 One-Molecule System. According to both the experimental and calculated data,10 it is generally accepted that the reaction of an alkyne molecule with bromine starts with the formation of a 1/1 Br2-alkyne π-complex. We have optimized the structures of these complexes for studied systems 1a-g. The general view of these complexes (π-C-I) is shown in Figure 2. Optimized structural parameters of these compounds are presented in Table 2 and in the Supporting Information. These species were assumed to have C2v symmetry for the alkynes with the same substituents (1a-d), with the Br2 molecule perpendicularly bound to the triple bond. Although the alkyne molecule is unsymmetrical (1e-g), the bromine atom attached to the triple bond (Br1) in π-C-I is shifted to 9178 J. Org. Chem., Vol. 70, No. 23, 2005
a
b
c
d
e
f
g
H H -4.03
Me Me -7.89
t-Bu t-Bu -7.45
Ph Ph -5.09
H Me -6.00
H t-Bu -6.21
H Ph -5.77
the carbon atom bound to H. In all studied cases, the π-C-I complexes have considerably lower values of Etotal than those for the reactants 1 + Br2. The formation of π-complexes for 1b and 1c is the most favorable. This is in good agreement with the idea that the triple bond is the most electron-rich in these species. No transition states have been found by us for reactions 1 + Br2 f π-C-I. According to the established mechanism, the next stage of the alkyne bromination reaction should be the formation of a carbocation (CC) from the π-C-I complex. We have optimized the structures of target carbocations CCa-CCg (Figure 3, Table 3). Unfortunately, we have not been able to find any transition states for the decomposition process, but the activation barrier (if present) must be close to the value of ∆E [Etotal(CC + BrΘ) - Etotalπ-C-I]. It should be noted that the relative energy of the calculated systems [CC + BrΘ] is very high in comparison with that for the related π-C-I complexes. Consequently, cleavage of the initially formed π-C-I complex with the formation of two charged fragments seems to be highly unrealistic, at least in the gas phase. However, as pointed out above, one can expect that these ions may form in solution. Thus, the analysis of the relationship of carbocation structures with R and R′ is required. Species CCa, CCb, and CCc derived from ethyne and the symmetrical bis-alkylethynes are the cyclic bromirenium cations. On the contrary, the structures of the cations formed from the unsymmetrical alkynes (CCe and CCg) strongly depend on the nature of the substituents R and R′. When R′ is phenyl, which can effectively stabilize the R-carbocationic center, CCg possesses the structure of the pure open vinyl cation. The influence of the Ph group is so great that CCd is also the open cation. It should be noted that CCf is unstable, and rearranges to the allyl cation CCf′ (Figure 3). According to our investigations, besides the unfavorable formation process of the carbocations CC discussed above, two other reaction pathways beginning with the π-C-I complex are possible (Scheme 2). Pathway I leads to the cis-dibromides, 3, through the transition state TS-1, shown in Figure 4. The main geometric parameters of TS-1 for the studied reactions
DFT Investigation of Alkyne Bromination Reactions
FIGURE 3. Calculated structures of carbocations CCa-e, CCg, and CCf′. TABLE 3. Calculated Relative Energies (∆E, kcal mol-1) of the Carbocations (CC) Formed from π-Complexes (π-C-I), and Calculated Values of ∆Ea That Are Evaluative Values of Eact of π-C-I f CC + BrΘ for the Decomposition Process R R′ ∆E ∆Ea
a
b
c
d
e
fb
g
H H 157.30 161.33
Me Me 130.83 138.73
t-Bu t-Bu 120.15 127.60
Ph Ph 114.04 119.13
H Me 141.09 147.10
H t-Bu
H Ph 115.94 121.71
∆E ) Etotal(CC + BrΘ) - Etotalπ-C-I. b This cation is unstable (see discussion). a
SCHEME 2. Possible Bromination Pathways Starting from π-Complex π-C-I
are presented in the Supporting Information, and the energy barriers are listed in Table 4. There are two different pathways for pathway I in which the alkyne is unsymmetric, except the reaction π-C-Ig f 3g. TS-1′s correspond to the process in which the carbon atom of C-R (i.e., C-H) is bound more closely to the Br atom
FIGURE 4. Calculated geometry of the found transition state in process I [TS-1′ (TS-1′′), general view].
than to the carbon atom of C-R′ (i.e., C-C) with the other bromine atom. Correspondingly, TS-1’s are the structures with the longer Br‚‚‚C-H contact. For simplicity, the sole transition state for the symmetric alkynes was named TS-1′. In the case of phenylacetylene (1g), only TS-1′ was found. According to the obtained geometric parameters, TS-1 possesses one sp2 carbon atom of the former triple bond, whereas in π-C-I both carbons have sp hybridization. The Br-Br distances in TS-1 (2.941-4.098 Å) are considerably longer than those in π-C-I (2.372-2.430 Å). One C-Br bond (1.943-2.094 Å) is noticeably shorter than the other C-Br distance (3.382-4.707 Å). These values allow TS-1 to be considered as the contact ion pair. It should be noted that the values of Eact for process I are high, but the presence of Ph groups lightens the formation of cis-dibromides 3. The activation barriers for the terminal alkynes in process I have lower values in comparison to those for the corresponding disubstituted alkynes. In the case of terminal alkynes, the reactions predominantly proceed through TS-1′, in which the stabilization of Cδ+ by an alkyl or phenyl group occurs. The second reaction is process II (in Scheme 2) that leads to 1,1-dibromides 4 through the transition state TS2, the intermediate cation (IC), and the transition state TS-3 (for the activation barriers, see Table 4). TS-2 (Figure 5, see geometric parameters in the Supporting Information) differs considerably from TS-1, although both structures form from the same π-complex, π-C-I. TS-2 is characterized by the shift of R (or R′) from one carbon atom of the former triple bond (C1) to the other (C2). At the same time, the Br-Br group is shifted from the center of the triple bond to the carbon atom (C1). The J. Org. Chem, Vol. 70, No. 23, 2005 9179
Zabalov et al. TABLE 4. Activation Barriers for the Reactions of Processes I, II, and III (kcal mol-1)
a
process/compound
a
b
c
d
e
f
g
R R′ E(act) π-C-I f 3 (TS-1′) E(act) π-C-I f 3 (TS-1′′) E(act) π-C-I f IC′ (TS-2′) E(act) π-C-I f IC′′ (TS-2′′) ∆E [EtotalIC′ - Etotalπ-C-I] ∆E [EtotalIC′′ - Etotalπ-C-I] E(act)IC′ f 4 (TS-3′) E(act)IC′′ f 4 (TS-3′′) E(act) 3 f 2 (TS-4′) E(act) 3 f 2 (TS-4′′) E(act) 2 f 3 (TS-4′) E(act) 2 f 3 (TS-4′′)
H H 45.43
Me Me 47.35
t-Bu t-Bu 48.40
Ph Ph 44.90
50.58
60.90
45.66
11.28
21.38
30.93
24.38
19.63
19.82
26.86
28.62
73.47
62.75
37.38
51.76
73.67
65.33
45.69
51.22
H t-Bu 42.10 56.30 39.44 54.00 23.01 22.55 22.47 23.09 65.58 68.91 61.49 64.82
H Ph 34.69
37.99
H Me 40.42 51.69 37.46 51.39 18.85 18.70 18.13 19.43 66.36 72.08 67.33 73.04
a
42.51 44.30 23.76 24.20 24.92 23.04 58.08 67.67 57.00 66.60
TS-1′′ was not found for this process.
SCHEME 3. Cis-Trans Rearrangement of Dibromoalkenes
FIGURE 5. Calculated geometries of the found transition states and the intermediate in process II [TS-2′ (TS-1′′), IC′ (IC′′), and TS-3′ (TS-3′′), general view].
migration of the H atom (TS-2′) is more favorable in comparison to the shift of R′ (Me, t-Bu, Ph) (TS-2′′) for all three studied substrates. The intermediate cation IC (Figure 5, see geometric parameters in the Supporting Information) is the local minimum on the potential energy surface (PES) for all studied cases, and represents a neutral particle in which one carbon atom of the former triple bond (C1) possesses a positive charge, whereas the bromine atom (Br2) is negatively charged. TS-3 (Figure 5, see geometric parameters in the Supporting Information) is characterized by the shift of the long bromine atom (Br2) toward the C1 atom of the former triple bond. IC′ and IC′′, as well as TS-3′ and TS-3′′, possess very similar geometries for the unsymmetrical alkynes. Unfortunately, we have not found any processes leading to trans-dibromides 4 (except the possibility of carbocation CC formation which should be ruled out). However, the cis T trans isomerization is a suitable source of transdibromides 2 (process III, Scheme 3). It should be noted that the process of the cis T trans isomerization of alkenes is actually complicated, and still not fully understood.43-45 We have analyzed process III by the DFT method, and found the ground-state saddle point on PES of this reaction; the found transition state 9180 J. Org. Chem., Vol. 70, No. 23, 2005
is consistent with the theory of this process,43-45 and corresponds to an ionic and highly polarizable zwitterionic state. As it was previously found by us for other transition states, there are two isomers for unsymmetric alkenes e, f, and g. The structures of TS-4 possess the dihedral angle Br1C1-C2-Br2, which is close to 90°. One carbon atom of the former double bond (C2) is sp2 hybridized. The extent of pyramidalization of the other carbon of the former double bond (C1) strongly depends on steric factors, and correlates with the strength of the C2-Br bond. Thus, for species TS-4′a and TS-4′e (see Figure 6) with a strong C2-Br bond (2.104-2.179 Å), a significant pyramidalization of C1 was found; compound TS-4′d with a weak C2-Br bond (3.309 Å) possesses an almost planar C1 atom. It should be especially noted that, in contrast to previous investigations (see, for example, the study of ethylene (CH2dCH2) and stilbene (PhCHdCHPh isomerization44)), the transition states TS-4 are the saddle points on PES. The comparison of the activation barrier values for processes I and II allows one to make some remarks on the gas-phase bromination of the alkynes. The presence of at least one hydrogen atom on the CtC bond, and the absence of a Ph group in alkynes 1, leads to formation of cis-dibromides 3b, 3c, 3d, and 3g as kinetically favorable products in the 1-Br2 system; in cases 1c and 1d, transdibromides are also expected. However, the presence of an H atom in alkynes 1a, 1e, and 1f promotes the formation of 1,1-dibromides because of the ease of the migration of hydrogen in comparison to the other studied substituents. This migration is necessary in this reaction pathway. The peculiar behavior of alkynes with Ph groups is probably related to its well-known ability to stabilize the positive charge on the R-carbon atom. (43) Brooks, B. R.; Schaefer, H. F., III. J. Am. Chem. Soc. 1979, 101, 307-311. (44) Han, W. G.; Lovell, T.; Liu, T.; Noodleman, L. ChemPhysChem 2002, 3, 167-178. (45) Viel, A.; Krawczyk, R. P.; Manthe, U.; Domcke, W. Angew. Chem. 2003, 115, 3556-3559; Angew. Chem., Int. Ed. 2003, 42, 34343436.
DFT Investigation of Alkyne Bromination Reactions
FIGURE 6. Calculated geometries of the found transition states in process III [TS-4′ (TS-4′′), TS-4′a, and TS-4′d are presented as examples].
SCHEME 4. Cleavage Processes for Dibromoalkenes 2 and 4
FIGURE 7. Calculated geometries of the found transition state in process IV [three different types of TS-5′ (TS-5′′), general view].
Thus, in general, three different addition products may form in the gas phase for the 1-Br2 system. Computational works based on the experimental data confirm the possibility of haloolefins (such as vinyl chloride, vinyl fluoride, 1,1-, cis-, and trans-1,2-difluoroethylenes) dissociating to give HHal as one of the products.28-32 The dissociation process of 1,1-dichloroethylene leading to Cl2 and C2H2 was also studied.27 Different pathways for dissociation processes were found, but all of them possess relatively high activation barriers (>90 kcal/mol). Bearing in mind the possibility of decomposition processes in the gas phase for dibromides 2-4, we have investigated PESs for these compounds (Scheme 4). It has been determined that there is no direct pathway from cis-dibromides 3 to the decomposition products 5-8. We have found processes IV and V that lead to 5-8, from 2 and 4, respectively (Table 5). The details of structures of TS-5 and TS-6 are shown in Figures 7 and 8. For the unsymmetrical dibromides 2 and 4, TS-5′ and TS-6′ led to the elimination of H-Br, whereas TS-5′′ and TS-6′′ led to the corresponding Alk(Ph)-Br. The main geometric parameters are listed in the Supporting Information. It should be noted that the nature of the substituents at the double bond governs the type of transition state structures, which are noticeably different. Thus, the dissociation process of trans-dibromoalkenes with a d C-H fragment (2a, 2e-g) leading to H-Br is character-
ized by the weakening of the C-Br bond in TS-5′ (type I, Figure 7), whereas the same processes leading to formation of Alk-Br pass through TS-5′ (2a-c) or TS5′′ (2e and 2f), which possess a weak C-Cd contact (type II, Figure 7). The formation of Ph-Br from 2d (TS-5′) and 2g (TS-5′′) is characterized by type III of TS-5, in which very close contact of Ph-Br was found (Figure 7). The same trends were also detected for TS-6 (Figure 8). We propose that the main reason for the possible existence of transition states TS-5 and TS-6 is the energetics of the process of the Hδ+ formation in comparison with the Cδ+t formation in the gas phase. Both of these processes (IV and V) require very high energy inputs. Formation of H-Br is considerably more favorable in comparison to the formation of Alk(Ph)-Br for both processes IV and V. Thus, the dissociation processes of 2 and 4 leading to Alk(Ph)-Br look physically unreal. Analysis of the Alkyne (1)-Br2 System in Chloroform Solution (as a Supermolecule). To estimate the solvent effects on the bromination process of alkynes, we studied the bromination reaction of 1b (π-C-Ib f 3b) in chloroform. The solvent influence was simulated by the addition of six molecules of CHCl3 to the system (cluster model). The found transition state (process I, Scheme 2) TS-1′b*6CHCl3 with the starting π-complex π-C-Ib*6CHCl3 and the product 3b*6CHCl3 are shown
TABLE 5. Activation Barriers for the Reactions of Processes IV and V (kcal mol-1) process/compound
a
b
c
d
e
f
g
R R′ E(act) 2 f 5 + 6 (TS-5′)
H H type I 64.18
Me Me type II 110.21
t-Bu t-Bu type II 70.04
Ph Ph type III 89.76
type I 72.71
type II 106.69
type II 87.74
type III 82.49
H Me type I 63.94 type II 111.27 type I 63.78 type II 106.24
H t-Bu type I 61.34 type II 98.88 type I 56.53 type II 92.27
H Ph type I 65.51 type III 84.82 type I 55.79 type III 93.87
E(act) 2 f 7 + 8 (TS-5′′) E(act) 4 f 5 + 6 (TS-6′) E(act) 4 f 7 + 8 (TS-6′′)
J. Org. Chem, Vol. 70, No. 23, 2005 9181
Zabalov et al. TABLE 6. Calculated Relative Energies (∆E, kcal mol-1) of 2:1 Br2-Alkyne π-Complexes (π-C-II) R R′ ∆E ∆Ea a
FIGURE 8. Calculated geometries of the found transition state in process V [three different types of TS-6′ (TS-6′′), general view].
in Figure 9. The structure of this transition state is close to that found for the gas-phase reaction (TS-1′b), with a minor elongation of Br1-Br2 (0.15 Å), Br2-C1 (0.10 Å), and Br2-C2 (0.08 Å); i.e., in chloroform solution the proposed process passes through a slightly more ionic transition state than that for the gas phase (TS-1′b). It should be noted that, as expected, the activation barrier was found to be considerably lower for the bromination reaction leading to cis-dibromoethylene (3b) in CHCl3 solution (supermolecule procedure, 23.57 kcal mol-1) than that in the gas phase (47.35 kcal mol-1). However, the calculated barrier (23.57 kcal mol-1) is higher than that found for the bromination process with two bromine molecules (see below). Analysis of the Alkyne (1)-2Br2 Two-Molecule System. As stated above, the examination of alkyne bromination with one bromine molecule and one molecule of the organic substrate is suitable for the description of
a
b
c
d
e
f
g
H H -8.11 -4.08
Me Me -13.61 -5.71
t-Bu t-Bu -13.77 -6.32
Ph Ph -10.94 -5.85
H Me -11.08 -5.08
H t-Bu -11.58 -5.37
H Ph -11.71 -5.94
∆E ) Etotalπ-C-II - Etotalπ-C-I.
the processes in the gas phase. However, the generally accepted mechanism of these reactions in solution involves the formation of a π-complex of the alkyne molecule with two molecules of bromine. This was confirmed by both experimental and calculated data for C2H2 (1a).10,16 Herges et al. have found that three different types of such complexes derived from 1a may exist.16 We have optimized the structures of 1-2Br2 π-complexes and found only one type of structure as local minima. The general view of these complexes (π-C-II) is shown in Figure 10. The structural parameters of these compounds are presented in the Supporting Information; relative energies are presented in Table 6. The geometry data verify the nature of these species as π-C-I complexes, with an additional bromine molecule that is weakly bound to the triple-bonded bromine atom (Br2). In all studied cases, π-C-II complexes have considerably lower values of Etotal than those for starting 1 + 2Br2 and π-C-I + Br2. No transition states have been found for reactions π-C-I + Br2 f π-C-II. It should be noted that although the formation of the carbocation (CC) with Br3Θ from the π-C-II complex is more favorable than the dissociation of π-C-I (CC + BrΘ), the activation barrier is very high (Table 7). No transition states for this type of process were found. As stated above, the main question in the mechanism of the alkyne bromination that is still unresolved is how cis- and trans-dibromoalkenes are formed from the π-CII complex if the formation of the carbocation (CC) is disadvantageous. Herges et al. suggested that in nonpolar solvent, the reaction may proceed without the formation of a cationic intermediate: “The Br3 anion forms a covalent bond with the cationic center. This tribromide adduct forms from a π-complex, and rearranges directly to the product (dibromide) without passing through a cationic intermediate.”16 However, only cis-dibromides
FIGURE 9. Results of the study of the bromination reaction as a supermolecule calculation. The found transition state (process I, Scheme 2) TS-1b′*6CHCl3, the starting π-complex π-C-Ib*6CHCl3, and the product 3b*6CHCl3. The main geometrical parameters for TS-1b′*6CHCl3 (bond lengths, Å; angles, deg): Br1-Br2 ) 3.106, Br1-C1 ) 2.001, Br1-C2 ) 2.755, Br2-C1 ) 4.150, Br2-C2 ) 3.859, C1-C2 ) 1.281, C1-R ) 1.501, C2-R′ ) 1.418; Br1-Br2-C2 ) 45.0, Br1-C1-C2 ) 112.3, Br1-C1-R ) 116.0, Br2-Br1-C1 ) 106.7, Br2-C2-C1 ) 94.0, Br2-C2-R′ ) 89.1, C1-C2-R′ ) 176.6, C2-C1-R ) 131.7.
9182 J. Org. Chem., Vol. 70, No. 23, 2005
DFT Investigation of Alkyne Bromination Reactions
FIGURE 10. Calculated geometries of π-complexes π-C-II derived from symmetrical and unsymmetrical alkynes (general views).
TABLE 7. Relative Energies (∆E, kcal mol-1) of [CC + Br3Θ] Systems, and Calculated Values of ∆Ea that Are Evaluative Values of Eact of the π-C-II f CC + Br3Θ Rupture Process R R′ ∆E ∆Ea
FIGURE 11. Calculated geometries of the found transition
a
b
c
d
e
fb
g
H H 114.94 123.06
Me Me 88.47 102.08
t-Bu t-Bu 77.79 91.56
Ph Ph 71.69 82.63
H Me 98.74 109.82
H t-Bu
H Ph 73.59 85.29
Θ a ∆E ) E b total(CC + Br3 ) - Etotalπ-C-II. This cation is unstable (see discussion).
SCHEME 5. Bromination Pathways Starting from π-Complex π-C-II
may form in this way. The formation of trans-dibromides was still in question. It should be noted (see Introduction) that these species at least form from sterically nonhindered alkynes during bromination. We have studied the possible pathways from π-C-II complexes to the dibromoalkenes, and three different processes were found (Scheme 5). Two of them (VI and VII) led to cisderivatives 3, but one (VIII) led to trans-dibromoalkenes 2.
states in processes VI, VII, and VIII [TS-7′ (TS-7′′), TS-8′ (TS8′′), and TS-9′ (TS-9′′), as well as TS-9′A, general views]. The main geometrical parameters for TS-9′A (bond lengths, Å): Br1-Br2 ) 3.520, Br1-Br3 ) 3.723, Br1-Br4 ) 5.235, Br1C1 ) 1.949, Br1-C2 ) 2.663, Br2-Br3 ) 2.588, Br2-Br4 ) 5.154, Br2-C1 ) 4.215, Br2-C2 ) 5.281, Br3-Br4 ) 2.569, Br3-C1 ) 3.426, Br3-C2 ) 4.087, Br4-C1 ) 4.232, Br4-C2 ) 4.186, C1-C2 ) 1.280, C1-R ) 1.087, C2-R′ ) 1.429; for angles see Supporting Information.
The transition states TS-7, TS-8, and TS-9 sufficiently differ from each other (Figure 11, see Supporting Information for geometric parameters). TS-7 structures may be considered as the analogues of TS-1 in which the additional bromine molecule stabilizes the BrΘ to form according to the geometrical parameters of the Br3Θ ion. Correspondingly, the new Br-C bond forms between the C2 atom and the Br2 atom, which is the nearest atom to the Br1 bound with C1. In contrast to TS-7, TS-8 possesses the Br-Br-Br fragment, which may not be considered as Br3Θ ion due to its nonlinearity. This fragment also attacks the cationic carbon center as it was found for TS-7, but the bond forms between the long bromine atom (Br4) and C2. Both processes VI and VII led to cis-isomers 3. According to the IRC analysis results, process VIII, which passed through TS-9 and led to the trans isomers 2, is characterized by the linear transfer of the Br-BrBr fragment with regard to the C2 atom. This transfer is accompanied by the rotation of the C2-R(R′) fragment around the C1-C2 bond. In general, there are not remarkable differences between the structures of TS-7′ and TS-7′′, TS-8′ and TS8′′, and TS-9′ and TS-9′′. It should be also noted that for tert-butylacetylene, the additional transition state (TS9′A) was also found (Figure 11). The comparison of activation barrier values for processes VI, VII, and VIII (Table 8) allows one to make several conclusions on the bromination of alkynes. The terminal alkynes 1a, 1f, and 1g react more easily with the bromine than do their disubstituted analogues. This conclusion is supported by the experimental data that show slow reaction rates for these reactions for sterically hindered derivatives.16 According to our data, phenylacetylene (1g) and diphenylacetylene (1d) give mixtures of the corresponding 2 and 3 products. This fact is J. Org. Chem, Vol. 70, No. 23, 2005 9183
Zabalov et al. TABLE 8. Activation Barriers for Processes VI, VII, and VIII (kcal mol-1)a process/compound
a
b
c
d
e
f
g
R R′ E(act) π-C-II f 3 (TS-7′) E(act) π-C-II f 3 (TS-7′′) E(act) π-C-II f 3 (TS-8′) E(act) π-C-II f 3 (TS-8′′) E(act) π-C-II f 2 (TS-9′) E(act) π-C-II f 2 (TS-9′′) E(act) π-C-II f 2 (TS-9A′)
H H 32.68
Me Me 30.26
t-Bu t-Bu 29.28
Ph Ph 22.87
21.27
26.01
27.91
26.97
13.54
19.90
27.91
23.67
H Me 27.34 35.75 21.65 25.89 16.45 19.42
H t-Bu 28.04 38.64 24.65 28.52 23.40 21.07 24.80
H Ph 14.64 41.91 17.66 32.62 25.60 15.93
a The DFT study of the bromination of various alkynes gave values of activation barriers for processes leading to cis-, trans-, and 1,1-dibromoalkenes without ionic intermediates.
consistent with the data of Uemura et al. and of Bianchini and co-workers.9,10 As expected, the reaction of sterically hindered 1c may lead to 3c (cis product) or 2c (trans product). The mixture of cis- and trans-dibromides may also form in the case of t-Bu-acetylene, but the trans derivative predominates. The other studied systems, i.e., dimethylacetylene (1b), methylacetylene (1e), as well as acetylene itself (1a), have to form the trans isomers 2 as a single product of the bromination reaction. This behavior also corresponds with experimental data.9,10 It should be noted that the values of the activation barriers found by Herges et al.16 for acetylene (1a) bromination resulting in 3a are 23.72 and 28.43 kcal mol-1 (depending on the type of starting π-complex). These magnitudes are sufficiently larger than those found by us (Table 8).
HBr) 5-8. The formation of HBr is the most favorable among possible cleavage processes. With regard to solution reactions, we have found that both trans- and cisdibromoalkenes (2 and 3) may ensue without the formation of ionic intermediates from a π-complex of two bromine molecules with the alkyne. The geometry around the forming double bond in dibromoalkenes strongly depends on the nature of the substituents at the triple bond. Among studied compounds, the formation of trans derivatives 2 is favorable for acetylene (1a), methylacetylene (1e), and dimethylacetylene (1b); sterically hindered alkynes 1c, 1d, 1f, and 1g may lead to a mixture of 2 and 3. In general, the terminal alkynes, as well as acetylene, undergo bromination more easily than their disubstituted analogues. These results are in good agreement with available experimental data.
Conclusions We have carried out the theoretical analysis of the bromination of various substituted alkynes. According to our calculation data, the gas-phase bromination of alkynes 1 leads, in general, to three different products: cisdibromoalkenes 3, 1,1-dibromoalkenes 4, and transdibromoalkenes 2 (as a result of isomerization of 3). The process of formation of 3 prevails for phenylacetylene (1g) and disubstituted alkynes (1b-d). The presence of a hydrogen atom in the alkynes 1a, 1e, and 1f promotes the formation of 1,1-dibromides 4. The pathway for the bromination of dimethylacetylene (1b) with one bromine molecule in chloroform (the supermolecule approach) is more favorable than the reaction in the gas phase, but is still high in comparison with that found for the bromination reaction with two bromine molecules. Dibromoalkenes 2 and 4 may decompose, resulting in bromoacetylenes and alkylbromides (phenylbromide or
9184 J. Org. Chem., Vol. 70, No. 23, 2005
Acknowledgment. We thank Dr. D. N. Laikov for placing at our disposal the program “PRIRODA”. We also thank the Research Computing Center of the Moscow State University for computational time. Many thanks are due to one of the reviewers for helpful discussions about the calculations of cis and trans isomerization of 1,2-dibromoalkenes. Supporting Information Available: Calculated main geometric parameters of starting alkynes (1), trans-dibromides (2), cis-dibromides (3), 1,1-dibromides (4), bromoacetylenes Br-CtC-R(R′) (5, 7) and R(R′)-Br (6, 8), and their energies. Calculated main geometric parameters of TS-1-TS-9. Cartesian coordinates of transition states and stable compounds together with the total energies and numbers of imaginary frequencies. This material is available free of charge via the Internet at http://pubs.acs.org. JO051085V
